Thrust and Conjugate Strike‐Slip Faults in the 17 June 2018 MJMA 6.1 (Mw 5.5) Osaka, Japan, Earthquake Sequence

Abstract

ABSTRACT The 17 June 2018 MJMA 6.1 (Mw 5.5) Osaka earthquake exhibits a large non–double‐couple component (∼26%), and its aftershock sequence shows a complicated spatial pattern. To better understand the ruptured faults, we relocate the earthquake sequence using P and S arrival times and waveform cross correlations and calculate the focal mechanisms of all MJMA≥2.5 (Mw≥2.3) earthquakes within three months after the mainshock using P‐wave first‐motion polarities and S/P amplitude ratios. Relocated aftershocks image several faults, the northeast‐striking strike‐slip fault, the north‐northwest‐striking reverse fault, and at least two small northwest‐striking features. P‐wave first motions of the mainshock indicate nearly a pure thrust mechanism. We deduce that the earthquake sequence started from a north‐northwest‐striking reverse fault and propagated to a northeast‐striking strike‐slip fault. The aligned strike‐slip aftershocks occurring in the vicinity of the northeast‐striking strike‐slip fault delineates the growth of several newly formed or reactivated northwest‐striking Riedel shears that are conjugated to the northeast‐striking strike‐slip fault.